Those of skill in the art to which this invention pertains will recognize the typical terminology used in the art. As a convenience for others whom may not be of skill in the art, the following description is provided so that they can better appreciate the limited nature of the prior art and advantages and importance of the present invention.
The term that describes or quantifies the branched nature of a polymer is called the degree of branching ("DB"). FIG. 6A is a schematic representation of terminal (T), linear (L), and dendritic (D) building blocks of AB.sub.2 type polymers. Polymers having a degree of branching approaching 0 are said to be linear (e.g., as shown in FIG. 6B) and those having a degree of branching approaching 1 are said to be dendritic (e.g., as shown FIG. 6D) or "maximally" branched. Anything in between these two extremes is said to be branched or hyperbranched depending upon the degree of branching (e.g., as shown in FIG. 6C). The formula that determines the degree of branching for the one-pot polymerization of an AB.sub.2 type monomer is given below: ##EQU1##
In equation 1 above, [D.sub.AB2 ] and [L.sub.AB2 ] represent mole fractions of dendritic and linear segments, that are incorporated into the polymeric backbone. See Holter D., Burgath A., Frey H., Acta Polymer 48, 30-35 (1997). The equation that determines the degree of branching for the one-pot polymerization of an AB/AB.sub.2 type polymerizations is given below: ##EQU2##
In equation 2, [D.sub.AB2 ], [L.sub.AB2 ] and [L.sub.AB ] represent mole fractions of dendritic and linear segments (FIG. 6E), respectively. See Frey, H.; Holter, D., Acta Polymerica 1999, 50(2-3), 67-76.
The physical properties of the polymers are determined by the size, shape, and peripheral chemistry (endgroups/B groups) of the polymer. The physical properties (e.g., crystallinity, solution viscosity and solubility) of dendritic macromolecules have been shown to be drastically different from their linear counterparts. See Hawker C. J.; Malmstrom E. E.; Frank C. W., Kampf J. P., Journal of the American Chemical Society, 1997, 119, 9903-9904; and Frechet J.; Hawker C. J.; Gitson I.; Leon J. W., Journal of Material Science-Pure Applied Chemistry 1996 A33(10), 1399-1425. The reason for this is believed to be that dendrimers are discrete molecules whose physical properties are determined by their unique globular shape (lack of intermolecular interactions) and the number of endgroups that occupy their periphery. This is in direct contrast to linear polymers whose physical properties are determined by chain entanglements (intermolecular interactions) and the structure of the repeat unit.
Linear Polymers in General
The majority of products in the plastics industry consist of linear polymers. This linear nature affords diverse physical and mechanical properties. The physical properties of linear polymers are highly dependent on molecular weight. Two examples of such properties are solubility and viscosity. As the molecular weight increases, the viscosity of the material increases. This can be a beneficial property if a viscous material is desired, but in many industrial processes, e.g., where injection molding is the processing method of choice, extremely viscous materials slow down the process and viscosity can become a limiting step of production. Also, as the molecular weight increases, the solubility decreases. If solubility resistance is the desired trait, then this is a suitable outcome. However, in other applications, e.g., the manufacture of coatings or films from liquids, low solubility leads to difficult manufacturing problems.
Thus, there is a need for materials that have much lower viscosity and much higher solubility than linear compounds, yet still retain the advantageous properties of linear compounds. These properties would make such materials ideal commercial candidates for use as additives or property modifiers for commercial coatings and injection molding processes, and thus useful in large volume industrial applications.
Dendritic Polymers in General
Dendrimers are "perfect," or maximally branched macromolecules that have drawn considerable attention in the last few years. See Frechet J.; Hawker C. J.; Gitson I.; Leon J. W., Journal of Material Science-Pure Applied Chemistry 1996 A33(10), 1399-1425; and Kim Y. H., Journal of Polymer Science 1998, 36, 1685-1689. From a materials standpoint, dendrimers have extremely useful properties, such as increased solubility and reduced viscosity at high molecular weights. However, attempts to make dendrimers have resulted in materials whose synthesis is extremely expensive, time consuming, and labor intensive (e.g., extremely difficult to purify).
Hyperbranched Polymers in General
The synthesis of hyperbranched polymers is an area of research that was discussed as early as 1952. See Flory P. J., J. Am. Chem. Soc. 1952, 74, 2718. More recently, interest in hyperbranched polymers has increased due to their possible use as alternatives to dendrimers. See Frechet J.; Hawker C. J.; Gitson I.; Leon J. W., Journal of Material Science-Pure Applied Chemistry 1996, A33(10), 1399-1425; and Kim Y. H., Journal of Polymer Science 1998, 36, 1685-1689. Thus far however, attempts to synthesize monomers necessary to make hyperbranched polymers have been costly and difficult, leading to the limited production of monomeric starting materials. The lack of available monomeric starting materials has slowed the bulk property testing of the hyperbranched polymers that could make them viable candidates for commercial applications. Nonetheless, much effort has been given to solve this problem and researchers have been trying to develop new methods and materials that utilize the hyperbranched approach to make cost-efficient, scalable hyperbranched polymers that mimic dendrimers, as evidenced by Frechet J.; Hawker C. J.; Gitson I.; Leon J. W.; Journal of Material Science-Pure Applied Chemistry 1998, A33(10), 1399-1425; Kim Y. H., Journal of Polymer Science 1998, 36, 1685-1689; and the United States Government Report (NIST) entitled "Workshop on Properties and Applications of Dendritic Polymers: Speaker Abstracts: Literature Review on Characterization, Modeling, and Applications" (July 9-10, 1998).
Moreover, to the extent that scalable quantities of monomers have been made, many of the hyperbranched polymers made from these monomers may not withstand severe chemical, mechanical, thermal and oxidative conditions. Thus, there is a great need for stable monomers and hyperbranched polymers, as well as convenient and cost efficient methods for their manufacture.
Polyetherimide Formation
Formation of an aromatic ether bond in an imide system via a nucleophilic aromatic substitution reaction can be done a number of ways. Typically (FIG. 8A), one takes an aromatic nitrophthalimide monomer 32 and reacts it with a bisphenolate salt monomer 31 in a high boiling polar aprotic solvents such as dimethylformamide ("DMF") or n-methylpyrrolidinone ("NMP") to produce high molecular weight ("MW") polymer. See White D. M.; Takekoshi T., Journal of Polymer Science 1981, 19, 1635-1658. The polymer 33 shown in FIG. 8A is a polyetherimide that is produced on commercial scale under the tradename Ultem.RTM. by General Electric Company. One of the problems with this synthesis is that in order to achieve high MW polymer, perfect stoichiometric ratios of the AA and BB monomers must be used and the conditions require strict adherence to anhydrous and oxygen free conditions which is difficult to achieve. Extrapolating from this chemistry, one can envision that monomers 34-39 could be synthesized. In U.S. Pat. No. 4,297,474, monomers 34 and 37 were synthesized and polymerized, but led to low molecular weight linear polymers 40 and 41. Monomers 35 and 38 could be synthesized to make hyperbranched PEIs, but it has been shown that the reaction rate of the 3- versus the 5-nitro is five (5) times lower, which in a one-pot polymerization procedure would lead to a polymer with a very low degree of branching. See Gosh M. K.; Mittal K. L.; Polyimides, Marcel Dekker, Inc., New York, N.Y., 1996; and Holter D., Frey H., Acta Polymer 1997, 48, 298-309. Further, the material would behave more like a linear polymer, have endgroups which would lead to decreased solubility of the homopolymer, and would be difficult to chemically derivatize. On the other hand, monomers 36 and 39 could be synthesized as well, but it has been previously shown that the chemistry doesn't work well when both phenols are on the same aromatic ring. See White D. M.; Takekoshi T., Journal of Polymer Science 1981, 19, 1635-1658. An electron transfer redox reaction occurs between electron-rich benzenediol dianions and highly electron deficient nitrophthalimides, thereby leading to low molecular weight polymers. See Gosh M. K.; Mittal K. L., Polyimides, Marcel Dekker, Inc., New York, N.Y., 1996.
More recently, Kricheldorf developed a catalytic method for nucleophilic aromatic substitution polymerization. See Gosh M. K., Mittal K. L., Polyimides, Marcel Dekker, Inc., New York, N.Y., 1996. In this scheme (FIG. 9), activated aromatic bishalo compounds (AA monomers) in the presence of a fluoride catalysts were reacted with bis(silylether)s of bisphenols (BB monomers) in place of bisphenol salts. The silyl ether is converted in situ to the corresponding phenoxide. The phenoxide undergoes substitution reaction, producing the polyarylethers and regenerating the halide anion. The volatile trisalkylsilylhalide by-product, boils off, driving the equilibrium toward high MW polymer. This procedure does not require preparation of oxygen-sensitive anhydrous bisphenol salts or the removal of the salt byproduct. Since the reaction takes place only at high temperature and in the presence of catalyst, it is contemplated that both functionalities could be placed on the same monomer creating an AB monomer. This would be advantageous from a polymerization standpoint since exact stoichiometric balance is assured in the monomer unit.
Bryant and St. Clair (FIGS. 10A-C) have employed the above process for the synthesis of AB-type polyetherimides monomers 42-45 and polymers 46 thereof, similar to that shown in FIG. 8A. See Bryant R. G., St. Clair T. L., Abstracts of the 4th International Conference on Polyimides, October/November 1991, Ellenville, N.Y., II-69. The inventors of the current invention followed this teaching of Bryant and St. Clair to make AB monomer 42 and 44 and found that the resulting monomers were unstable which led to polymers of low molecular weight. Therefore, stable monomers that could be purified to 99+% were not taught by conventional methods.